Electron beam (ebeam) emitting apparatus have been used over the years to treat moving webs for various purposes, such as to sterilize a web of packaging material, polymerize web material, cure a coating on a web, etc. Of particular interest here is the utilization of such an ebeam to sterilize packaging material. Sterilization of the packaging material typically leads to a longer shelf life of the product that is packaged therein. In the food packaging industry, it is well-known to form various open receptacles from a web made of various materials which are subsequently filled with a product, e.g. a beverage, and then sealed transversely or sealed by adhering another material to the top edge of the receptacle. The receptacles are then separated at spaced-apart locations therealong, creating a succession of individual closed containers. Packaging applications of this kind are typically referred to as either Horizontal Form Fill Seal (HFFS) or Vertical Form Fill Seal (VFFS). In HFFS machines, the forming and filling steps are accomplished as the web is traveling horizontally, or parallel to ground plane. HFFS machines are typically used for cup based products. In VFFS machines, the forming and filling steps are accomplished as the web is traveling vertically, or perpendicular to ground plane. VFSS machines are typically used for packs, stick packs, and bag based products.
As used here, the term “packaging material” generally means packaging is substrates such as polymer films, carton board and multilayer laminates in web and strip form that may be made into product packages such as packs, cartons, pouches, bags, stick-packs, sachets, cups, gable top containers, bag-in-box containers and other containers that may be filled with a food, beverage, pharmaceutical, medical device or other article which should be maintained in an aseptic condition prior to use, as well as such packages already formed from a substrate, both when the packages are empty and already filled with products.
When being sterilized, the packaging material may have to be moved through the irradiation zone of an ebeam emitter in a non-continuous fashion in order to accommodate a processing step being performed on the packaging material by an upstream or downstream machine that requires intermittent slowing or stopping of that material. For example, if the packaging material is a substrate being fed to a forming machine, the substrate movement through that zone may be slowed periodically as that machine transforms the substrate into a succession of containers of one kind or another. Likewise, if the packaging material being sterilized comprises a succession of open containers traveling on a conveyor to a filling machine, the conveyor may have to be stopped briefly or at least slowed while each container is being filled with product.
The dose applied to the packaging material exposed to an ebeam may be calculated as follows:
                    Dose        =                              (                          K              S                        )                    ⁢          i                                    (        1        )            
Where:
K=an efficiency constant which depends on multiple parameters of a specific application including: accelerating voltage, distance from electron beam window to is packaging material, other factors affecting the efficiency of electrons reaching the packaging material
S=speed
i=ebeam emitter filament current
When the packaging material is required to move non-continuously through the emitter's irradiation zone due to such processing constraints, some portions of that material spend more time in the irradiation zone than others and thus receive a larger ebeam dose than if the electron beam source was kept at constant current and parameters affecting K as described above were kept constant.
In order to maintain a dose delivery to the packaging material that falls within the required range for a particular application it is be desirable to modulate the effective dose delivery as a function of the speed S. However, modifying the ebeam dose applied to the packaging material necessitated by the non-continuous advance thereof may have an adverse effect on the ebeam emitter.
More particularly, a typical electron beam source or emitter includes a vacuum chamber containing an electron generator for generating electrons. The vacuum chamber has an exit window where the electrons, after being accelerated, exit the chamber through that window and are directed to the packaging material being treated. The emitter exit window includes a thin foil which is usually supported by a perforated support plate. During normal operation of the emitter, the exit window will absorb some amount of the electron energy which results in heating of the foil. The amount of energy absorbed by the window is proportional to the current of the beam and inversely proportional to the voltage of the beam. If that heating is excessive, the foil may fail. Since the foil constitutes part of the wall of the vacuum chamber, such a failure will cause the vacuum chamber to lose its vacuum, thereby disabling the emitter.
The thermal stresses on the exit window are particularly great if the emitter output power is cycled rapidly, thereby creating more dramatic thermal cycling of the foil than if the emitter were operated at a constant power output. A known disadvantage of the prior art arrives when the emitter is being used to treat packaging material which is advanced in a non-continuous manner. In such environments, the integrity of the emitter's exit
window is directly affected by the ability to manage the heating and cooling cycles of the window.
During web packaging sterilization, the web is drawn from a roll rotatably mounted to a roll stand and conducted to the packaging machine by way of a shielded irradiation compartment or tunnel. The web enters the tunnel through an entry slot and is guided through an irradiation zone in the tunnel where at least one surface of the web is exposed to a beam of electrons of sufficient energy and for a sufficient time to kill or inactivate microorganisms so as to disinfect the surface of the web. The web leaves the irradiation tunnel through an exit slot coupled to the packaging machine in which the web is formed into receptacles, filled, and sealed as described above. An example of such apparatus for sterilizing web used for product packaging is described, for example, in U.S. Pat. No. 7,417,239, entitled METHOD AND DEVICE FOR ELECTRON BEAM IRRADIATION, the contents of which are hereby incorporated by reference.
As seen from the above-incorporated patent, when the sterilizing apparatus is in is operation to remove microorganisms and other particles, it is desirable to maintain a flow of cool air which flows from the packaging machine into the sterilizing apparatus counter to the direction of web travel through the irradiation tunnel. This air flows to exhaust ports located near where the unsterilized web enters the tunnel. The same air flow also sweeps out any ozone that may be present formed by interaction of the electrons with air.
Also, in aseptic applications, such as in the packaging of certain food and pharmaceutical products, before each web run, it is common practice during a sterilization-in-place (SIP) cycle to sterilize the interior surfaces of the sterilizing apparatus and packager by spraying a fluid sterilant such as peracetic acid (PAA) or vaporized hydrogen peroxide (VHP) into the incoming air stream so that the sterilant is circulated throughout the interior of the apparatus including its irradiation tunnel. The sterilant condenses on all interior surfaces of the tunnel thereby sterilizing those surfaces. The sterilant is usually removed by subsequently circulating through the apparatus hot air whose temperature is above that of the sterilant's vaporization temperature and the resulting vapor is drawn out of the apparatus through the exhaust ports.
A known disadvantage of the prior art is that some of that SIP sterilant may damage the electron beam system if substantial amounts of vaporized sterilant come into direct contact with the electron beam window.